1. Field of the Invention
The present invention relates to the surface modification of dry monolithic aerogels.
2. Description of Related Art
Monolithic aerogels are a special class of open-cell porous materials derived from the supercritical drying of cross-linked inorganic or organic gels. By today's standards, typical aerogels are porous materials in which all structural entities (i.e., pores, particles) are smaller than 5000 .ANG.. Such materials have ultrafine pore sizes of less than 5000 .ANG., continuous porosity, high surface areas of typically 400-1000 m.sup.2 /g, and a microstructure composed of interconnected colloidal-like particles or polymer chains with typical characteristic diameters of less than 500 .ANG.. This microstructure is responsible for the exceptional optical, acoustic, thermal, and mechanical properties of such aerogels. In most instances, it is essential to obtain such dried gels in a monolithic state, i.e., free of cracks.
Silica aerogels are the most extensively described aerogel materials in the scientific and patent literature. Aerogels of transition metal oxides, in particular, are not as well described, and these aerogels are expected to possess some properties that are not possible with silica aerogels due to the presence of the transition metal. The new characteristics of the aerogels will produce interesting new materials for optical, magnetic, and catalytic applications.
The first aerogels were translucent pieces of porous silica glass made by S. S. Kistler (U.S. Pat. No. 2,249,767). Kistler's aerogels are prepared by forming silica hydrogels, which are exchanged with alcohol and dried. The alcohol is supercritically extracted in the drying process, and the resulting aerogel has a density of about 0.05 g/cm.sup.3. Kistler's process is time-consuming and laborious, and subsequent advances in the art have reduced the processing time and increased the quality and porosities of aerogels.
Other related art discusses the production of metal oxide aerogels other than silica aerogels. Teichner et al., in Advances in Colloid and Interface Science 5:245-273 (1976), provides a general discussion of metal oxide aerogels, including oxides of silicon, aluminum, titanium, zirconium, magnesium, nickel, copper, and molybdenum. Lynch (U.S. Pat. No. 3,977,993) discusses a modified Kistler method for making metal oxide aerogels. These aerogels are made by preparing a hydrogel, exchanging the water in the gel with an organic solvent, and then supercritically extracting the organic solvent. The Lynch patent does not discuss the peculiar problems in using different metals and the process conditions necessary to ensure that the resulting aerogels form large, transparent, intact (monolithic) solids.
European Patent No. 0382310 by Enichem discusses a process for preparing monoliths of metal oxide aerogels. The process comprises an acidic hydrolysis of a metal alkoxide, the gelation of the resulting colloidal solution, and the supercritical drying of the gel. The patent recognizes the difficulty in obtaining monolithic aerogels with metals other than silicon. The patent addresses the problem by adding a powder of a metal oxide to the colloidal solution at the end of hydrolysis, before gelation.
Aside from metal oxide aerogels, organic aerogels result from the reactions of certain organic compounds, for example (1) resorcinol with formaldehyde (known as RF aerogel), (2) melamine with formaldehyde (known as MF aerogel) and (3) phenolic-furfural with propanol. Such aerogels can be prepared in monolithic form and have been employed in double layer capacitors.
Many applications of aerogels require exposure to water or atmospheric moisture. Normally aerogel materials have a large affinity to absorb liquids such as water due to their high porosity with pores open to the surface. However, present aerogels are prepared either hydrophilic (i.e., absorb liquid water) or are only temporarily hydrophobic (i.e., shed liquid water). Methods are needed to either initially prepare hydrophobic aerogels, or treat the dried and/or fully prepared aerogels to achieve permanent hydrophobicity at ambient conditions as well as over a range of temperature and pressure conditions.
As early as the 1970's, fluidized beds of highly dispersed oxide and mixed oxide particles have been treated with various organic silicon compounds and controlled amounts of steam to produce products having hydrophobic properties. See, for instance, U.S. Pat. No. 3,873,337, where Laufer et. al. describe the fluidized bed treatment of highly dispersed, relatively low surface area (130 m.sup.2 /g), low porosity oxides with gaseous dialkyldichlorosilane and water in an atmosphere of CO.sub.2. However, such treatments do not consider the problems encountered to hydrophobize the present day relatively thick, highly porous, high surface area, monolithic aerogels that are essentially free of dispersed particles.
Even the modification of hydrophilic surfaces of such monolithic, low density aerogels with methanol vapor by Lee et al., "Low-density, hydrophobic aerogels," Journal of Non-Crystalline Solids, vol. 186 (1995), has produced hydrophobic aerogels for a relatively short period. The very high porosity of such dried aerogels, especially pores on open surfaces having an unusually high affinity to water, contributes to the problem of preparing permanently hydrophobic aerogels. Since many of the present-day applications of the subject aerogels require a wide variety of atmospheric exposures, the search continues to produce a monolithic, transparent and thick aerogel having permanent hydrophobicity at ambient conditions, yet still retain such properties over a wide range of temperature and pressure conditions.